1. Field of the Invention
The present invention relates to ferroelectric composites, and more particularly, to a method for preparing such composites employing sol-gels.
2. Description of Related Art
The use of ferroelectric ceramics with voltage-tunable dielectric properties for electronically-scanned array (ESA) antenna applications has been severely limited due to a lack of suitable fabricated materials. The properties needed for these applications at high frequencies (about 10 to 80 Giga Hertz, GHz) include a low dielectric constant (less than about 100), a low loss tangent tan .delta. (less than about 0.010), a large electric field-induced change in refractive index (ranging from 0.25 to 1.2) with a maximum applied electric field of less than 50 kV/cm, and a high dielectric breakdown strength (about 80,000 to 100,000 V/cm).
Composites have been under development with the idea of achieving a low-loss tangent by impregnating a porous structure with a low-loss polymer. These methods have involved the preparation of a ceramic powder, mixing it with a binder, burning out the binder to achieve a porous structure, sintering (firing), and then infiltrating the structure with a polymer. The difficulty with this conventional approach is that the homogeneity of the microstructure and resulting dielectric properties are not controllable within the desired limits of .+-.2% rms. Further, the polymer-impregnated ferroelectric composite exhibits loss at the operational frequencies, which is due, in part, to the presence of impurities introduced by conventional ceramic processing.
Prior art approaches to provide materials with voltage-variable dielectric properties include two basic methods, neither of which is entirely satisfactory for the high-frequency ESA antennas. One method involves the use of porous ferroelectrics such as barium strontium titanate ((Ba,Sr)TiO.sub.3 ; BST). The porous BST suffers from the separation of the individual particles with higher pore volumes and intrinsic difficulty in achieving a homogenous microstructure which controls the range of resulting dielectric properties. The separation of the individual particles is described as a lack of "connectivity" which is a term used to describe the degree of touching or connection between the particles. When the particles become more disconnected, then the applied field moves more into the polymer filler, which reduces the electric field-induced change in refractive index of the composite. The field must then be increased to obtain at least some electric field-induced change in refractive index, which leads to a dielectric breakdown or catastrophic arcing in the BST composite.
The other method of achieving some electric field-induced change in refractive index is that of employing a honeycomb, or similarly structured, composite. The difficulty with this method is that the pores are relatively quite large, so that the effective range is limited to lower frequencies, less than about 5 GHz.
Thus, a ferroelectric composite having a controlled, small-pore size is required, together with a process for making the same, for use at higher frequencies, on the order of 10 to 80 GHz.